Nutrient availability and indirect (biotic) defence in a Malaysian ant-plant.

Tropical plants of different genera defend themselves via symbiotic ant colonies, which are housed and often nourished by their host plant. Many studies deal with the defensive effects of the ants, but none has linked the plants' investment in this type of defence to the size and defensive efficacy of the symbiotic ant colony. We show here that ant-food production by the obligate myrmecophyte, Macaranga triloba, is limited by nutrient supply. The colony size of the ants in untreated plants (which had not been affected by experiments in advance of colony collection and determination of food body production) was significantly correlated with the amount of food produced by their hosts, and the plants' level of leaf damage was significantly and negatively correlated with the number of inhabiting ant workers. Our study provides the first field data that show that nutrient availability can directly influence a myrmecophyte's investment in its ants. Further studies are needed to evaluate whether soil nutrient contents in general can be a factor that limits the ability of myrmecophytes to defend themselves indirectly by nourishing symbiotic ants.